This invention relates generally to a light emitting chip and to an optical communication apparatus using the light emitting chip.
The semiconductor laser has been used as a light emitting source in audio discs, video discs, optical communications, and similar devices.
A buried-hetero structure (hereinafter abbreviated to "BH") has been developed as one of the structures of the semiconductor laser chip of the type described above. For example, the magazine "Electronics Materials", published by Industrial Research Association, April, 1979, pages 26-28 describes a GaAs-GaAlAs system BH semiconductor laser, and the same magazine also describes in the April issue, 1983, page 92, an InP-InGaAsP system BH semiconductor laser. A visible light band semiconductor laser (wavelength=0.7-0.9 .mu.m) formed by the GaAs-GaAlAs system has substantially the same BH laser chip structure as that of a long wave band semiconductor laser (wavelength=1.2-1.6 .mu.m) formed by the InP-InGaAsP system.
Now, the long wave band semiconductor laser will be explained by way of example.
FIG. 1 illustrates the structure of the BH type semiconductor laser developed by the inventors of the present application prior to the present invention. A buffer layer 2 consisting of low concentration n-type type InP, an active layer 3 (d=0.15 .mu.m) consisting of undoped InGaAsP, a clad layer 4 consisting of p-type InP and a cap layer 5 consisting of p-type InGaAsP are sequentially formed by a liquid phase epitaxial process on an n-type InP single crystal substrate 1 having a (100) crystal plane on its main surface. The total thickness of these four epitaxial layers is from approximately 3 to 4 .mu.m. Thereafter, this multi-layered grown layer is removed by the customary photolithographic process with an etching solution such as bromoethanol so that the cap layer 5 is left in a striated form having a width of from 5 to 6 .mu.m. This striated portion is disposed in such a manner as to extend in the direction of &lt;110&gt; axis of the crystal so that the edge surface of the active layer 3 becomes a (110) cleavage to improve the light emitting efficiency. In consequence, the crystal exhibits anisotropy with respect to the etching solution described above, and the portion extending over the active layer 3, the clad layer 4 and the cap layer 5 has an inverted truncated triangle cross-section, that is, an "inverted mesa" structure. The side plane forming this inverted mesa structure (hereinafter referred to as the "inverted mesa plane" for the sake of description) becomes a (111) crystal plane on which the In atom appears.
The lower part of the inverted mesa structure of the strip portion continues a forward mesa structure described by gentle curves B and C as shown in the drawing, and the boundary between the inverted mesa structure and the forward mesa structure becomes a portion having the smallest width (hereinafter called a "neck 7" for the sake of description). The portion 6 encircled by a dotted line will be called a "double hetero structure" for the sake of description.
In the drawing, symbol B represents the (111) plane on which the P (phosphorus) atom appears, and symbol C represents the (100) plane or the plane in the proximity of the former. The active layer 3 is formed above this neck portion (see "Electronics Materials", published by Industrial Research Association, April, 1983, page 92, FIG. 7).
After this mesa etching, the portion which has been etched and which has become recessed is buried by laminating a blocking layer 8 covering the side surface of the active layer 3 and consisting of p-type InP, a buried layer 9 consisting of n-type InP and a cap layer 10 consisting of n-type InGaAsP. Zn is diffused into the mesa portion 9 so as to reach the intermediate portion of the clad layer 4, and a p+ ftype ohmic contact layer 11 is defined. Furthermore, electrodes 12 and 13 are disposed at predetermined positions on the mesa portion and on the reverse of the substrate 1, respectively. The substrate 1 is then divided in a predetermined manner into laser chips 14 of several hundred .mu.m square. Reference numeral 15 in the drawing represents an insulating film (SiO.sub.2 film).
When used as the light source for optical fiber communication, the semiconductor layer chip must have characteristics such that it has a low operating current, that a large optical output can be sent into the optical fiber, that modulation can be made up to a high frequency, that the spectral wide is small, and that the change of the optical characteristics with the temperature change is small. The BH laser chip has been employed so as to satisfy these requirements.
The applicants have made intensive studies in order to develop a laser chip which is operative at a low operating current (low threshold current I.sub.th) and has high performance. The process of these studies will be described briefly.
First of all, the inventors believe that since the threshold current (I.sub.th) of the semiconductor laser depends only upon the width and thickness of the active layer, the position of the active layer at the mesa-like double hetero junction is a mere parameter that decides the width of the active layer.
Therefore, the inventors have developed a technique which can obtain the width of the active layer in a desired width range (e.g. from 1.1 to 1.9 .mu.m) with a high yield, and can locate the center position of the active layer having a thickness of 0.15 .mu.m within a range extending from a position deviating by 0.5 .mu.m towards the upper side from the neck (hereinafter called the "positive side") to a position deviating by 0.2 .mu.m towards the lower side from the neck (hereinafter called the "negative side").
However, in the BH laser chip described above, a problem is encountered in that, since it is difficult to control the position, (i.e. height) and width, of the active layer 3 and the width of the neck 7, they readily tend to deviate from the predetermined values so that the threshold current (I.sub.th) increases while the production yield drops.
The applicants assume that the reason for this is as follows. Since the active layer is arranged at the position close to the neck having the smallest width, the width will change drastically if the position of the active layer moves only slightly upward from the neck.
To cope with this problem, the applicants have produced the BH laser chip by arranging the center position of the active layer 3 above the neck 7 so that the change of the width remains unremarkable even if the position of the active layer changes in the vertical direction to some extent.
However, many BH laser chips produced in this manner still exhibit greater threshold current values (I.sub.th) than the rated value.
The present invention is completed on the basis of the studies described above.